cmd/cover: fix TestHtmlUnformatted in module mode

Updates #30228

Change-Id: Id9dffa6c805ac630945bac8febe342ce633626c6
Reviewed-on: https://go-review.googlesource.com/c/162830
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/cover/cover_test.go

@@ -448,6 +448,10 @@ func TestHtmlUnformatted(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if err := ioutil.WriteFile(filepath.Join(htmlUDir, "go.mod"), []byte("module htmlunformatted\n"), 0444); err != nil {
+		t.Fatal(err)
+	}
+
 	const htmlUContents = `
 package htmlunformatted
 
@@ -475,6 +479,7 @@ lab:
 
 	// testcover -html TMPDIR/htmlunformatted.cov -o unformatted.html
 	cmd = exec.Command(testcover, "-html", htmlUProfile, "-o", htmlUHTML)
+	cmd.Dir = htmlUDir
 	run(cmd, t)
 }